In recent memory hard disk drives, high storage capacity and miniaturization have been demanded. In order to increase the recording density, it has been strongly urged to lower flying height of a magnetic head and to reduce the unit recording area. Along with this trend, the surface quality required after polishing has become severely assessed every year even in a process for manufacturing a substrate for a memory hard disk. Because of the lowering of flying height of the magnetic head, the surface roughness, the fine waviness, the roll-off and projections are reduced, and because of miniaturization in unit recording area, the sizes and depths of the scratches and pits are decreased.
In order to meet such requirements, there has been proposed a polishing composition containing silica particles as abrasive particles having a specified particle size distribution (for instance, Japanese Patent Laid-Open No. 2002-30274).
It is disclosed in the publication that according to the above-mentioned polishing composition, since colloidal silica particles having different mono-modal number particle size distributions are contained, an aluminum disk substrate having small average waviness and a few surface defects can be obtained.
However, in the above polishing composition, a sufficient reduction in the surface roughness of the substrate for a memory hard disk cannot yet be achieved from the viewpoint of TMS-Ra which can evaluate the surface roughness of the entire substrate.